| Contents Notes |
1. Waste foundry sand contains high concentrations of lead, copper and zinc, constituents of brass. Lead ranged from 1400 mg/kg to 5800 mg/kg in the waste sand from the landfill. This material fails the TCLP test; the lead in the TCLP extract is over 5 mg/L. 2. When iron metal is mixed with the waste sand, there is no reaction with the lead; the lead is in no way altered or immobilized. Lead-contaminated waste sand mixed with about ten percent iron will pass the TCLP test; the lead in the TCLP extract will be less than the regulatory limit. Zero valent iron will reduce lead (II) ions, removing them from solution. Copper will also be reduced by iron metal, but not zinc. 3. The key fact supporting the conclusions of this study is that waste sand removed from the landfill after several years burial often fails the TCLP test. This is due to the oxidation of iron so that zero valent iron is no longer available to reduce the lead. Both lead and copper concentrations in the TCLP extracts of this hazardous material indicate that metallic iron is no longer active. If it is still present, it must be inacitvated by a coating of oxides. 4. Iron oxides, especially hydrous ferric oxide, can adsorb lead. Under some circumstances enough lead can be adsorbed so that lead in the TCLP extract is below 5 mg/L. However, adsorption alone is not a reliable method of permanently immobilizing lead. Adsorption is too dependent on sample history, pH, and other factors to be trusted for permanent immobilization. There is no certainty that the material will continue to pass the TCLP test with time. This contention is confirmed by TCLP tests of waste sand collected from the Nacogdoches landfill. 5. The distinction between whether only adsorption is occurring or whether oxidation/reduction is also occurring can be made by direct measurement of redox potential or by examining copper concentrations in the TCLP extract. Adsorption does not decrease copper concentrations in the TCLP extract nearly as much as reduction. |
